Reset controllers and reset control circuits are known. Such controllers may be found within any microprocessor and microprocessor based circuit subject to disruption caused by power supply fluctuations. Areas of application of reset controllers may include consumer electronics equipment such as home computers or any other application where erroneous results may be associated with supply voltage fluctuations.
Reset controllers may be used within microprocessor circuits to generate a reset signal upon loss of power or upon a detected fault condition. Detected fault conditions may include out-of-spec supply voltages or transient on a supply voltage power bus that may affect microprocessor operation.
Prior art attempts to build reset controllers have included the stand-alone use of timing circuits such as the 555 timer or the Maxim Integrated Products, Inc. line of supervisory circuits (e.g. the MAX690). Reset controllers constructed exclusively of 555 timers work well on start-up by providing a time delay, deferring the start of data processing, until such time as steady-state conditions have been reached.
Problems arise, on the other hand, with reset controllers constructed of 555 timers under conditions of out-of-spec. supply voltages and high speed transients. A 555 timer may not function (time) properly with a supply voltage of less than 4.5 volts. A 555 timer may also fail to detect high speed transients (glitches greater than 1 MHz in frequency).
Reset controllers using a MAX690 (provided by Maxim Integrated Products) supervisory circuit performs better than a 555 timer under conditions of out-of-spec supply voltages. Included within the MAX690 is an internal reference voltage supply that may be used in conjunction with external resistors to detect out-of-spec. supply voltages.
A reset controller using the MAX690 supervisory circuit may achieve a desired reset time delay using an external capacitor. A reset controller may also achieve the desired time delay through use of additional 555 timers.
While the MAX690 performs considerably better than a 555 timer in detecting low voltage conditions the MAX690 is of limited value in detecting high speed transients. The response time of the MAX690, in fact, is listed as being 100 .mu.s. Because of the importance of supply voltage control for microprocessors and microprocessor circuits a need exists for a reset controller that can detect low voltage conditions and glitches of less than 2.0 .mu.s.